1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive monitoring a supply current to protect motor driver circuits.
2. Description of the Prior Art
FIG. 1 shows a prior art disk drive comprising a disk 2 rotated about a center axis by a spindle motor 4. The spindle motor 4 comprises a number of windings (e.g., φA, φB, φC) configured to form a multi-phase motor. Each winding (φA, φB, φC) has a first end connected to a center tap and a second end connected to a respective pair of switches (6A, 6B, 6C). Switching logic 8 generates a control signal 10 for controlling the switches (6A, 6B, 6C) to selectively connect the second end of the windings to a supply voltage 12 or to ground according to a commutation sequence. A current flowing from the supply voltage 12 through the windings to ground generates a torque which causes the spindle motor 4 to rotate. A signal generator 14 generates a modulated periodic signal 16 (e.g., a pulse width modulated (PWM) signal) which controls the switches (6A, 6B, 6C) to control the amount of current flowing through the windings (the amount of torque) and thereby the speed of the spindle motor 4. The modulated periodic signal 16 may control either the top or bottom switch in each pair of switches (6A, 6B, 6C).
The prior art disk drive of FIG. 1 further comprises a voice coil motor (VCM) 18 for rotating an actuator arm 20 about a pivot in order to actuate a head 22 radially over the surface of the disk 2. A VCM driver circuit 24 comprising an H-bridge driver drives current from the supply voltage 12 through a voice coil of the VCM 18 to ground. The current following through the voice coil to ground generates a torque which rotates the actuator arm 20 about the pivot, wherein the direction of rotation is determined by the direction of current flowing through the voice coil. The H-bridge driver in the VCM driver circuit 24 is typically operated in a linear or PWM mode.
Excessive current due, for example, to a motor winding shorting to a ground source can damage the motor driver circuits. A short may be caused by a lead in the printed circuit board assembly (PCBA) contacting the case of the disk drive or bracket assembly of the host computer. The motor winding may also short to the magnets within the motor. In the prior art disk drive of FIG. 1, each pair of switches (6A, 6B, 6C) comprises a high-side current sensor (26A, 26B, 26C) for sensing the magnitude of the current (IA, IB, IC) flowing through the respective switches and a low-side current sensor (27A, 27B, 27C) used for current regulation in a servo loop. A current limit circuit 28 deactivates the modulated periodic signal 16 through AND gate 30 if the magnitude of the current sense signals (IA, IB, IC) exceeds a threshold, thereby preventing excessive current from damaging the switches (6A, 6B, 6C). Although this technique helps protect the driver circuits of the spindle motor 4, the driver circuits of the VCM 18 remain susceptible to damage from excessive current. This technique also increases the cost and complexity of the disk drive since it requires a separate current sensor for each pair of switches (6A, 6B, 6C) of the spindle motor 4. Current sensors could also be employed to protect the driver circuits for the VCM 18 (similar to the current sensors (26A, 26B, 26C) that protect the driver circuits for the spindle motor 4); however, this solution would also increase the cost and complexity of the disk drive.
There is, therefore, a need for a disk drive with improved current limit protection of motor driver circuits.